A Meteoroid Hits the Moon

June 13, 2006: There's a new crater on the Moon.
It's about 14 meters wide, 3 meters deep and precisely one
month, eleven days old.

NASA
astronomers watched it form: "On May 2, 2006, a meteoroid
hit the Moon's Sea of Clouds (Mare Nubium) with 17 billion
joules of kinetic energyâthat's about the same as 4 tons of
TNT," says Bill Cooke, the head of NASA's Meteoroid Environment
Office in Huntsville, AL. "The impact created a bright
fireball which we video-recorded using a 10-inch telescope."

Lunar
impacts have been seen before--"stuff hits the Moon all
the time," notes Cooke--but this is the best-ever recording
of an explosion in progress:

The
video plays in 7x slow motion; otherwise the explosion would
be nearly invisible to the human eye. "The duration of
the fireball was only four-tenths of a second," says
Cooke. "A student member of our team, Nick Hollon of
Villanova University, spotted the flash."

Taking
into account the duration of the flash and its brightness
(7th magnitude), Cooke was able to estimate the energy of
impact, the dimensions of the crater, and the size and speed
of the meteoroid. "It was a space rock about 10 inches
(25 cm) wide traveling 85,000 mph (38 km/s)," he says.

If
a rock like that hit Earth, it would never reach the ground.
"Earth's atmosphere protects us," Cooke explains.
"A 10-inch meteoroid would disintegrate in mid-air, making
a spectacular fireball in the sky but no crater." The
Moon is different. Having no atmosphere, it is totally exposed
to meteoroids. Even small ones can cause spectacular explosions,
spraying debris far and wide.

"That's
what we're trying to find out," says Cooke. "No
one knows exactly how many meteoroids hit the Moon every day.
By monitoring the flashes, we can learn how often and how
hard the Moon gets hit."

The
work is underway. Using a computerized telescope built by
Rob Suggs and Wesley Swift of the Marshall Space Flight Center,
Cooke's group is monitoring the night side of the Moon "as
often as ten times a month, whenever the lunar phase is between
15% and 50%."

During
a telescope test last November 7th, Suggs and Swift recorded
an explosion on their very first night of observing. A piece
of debris from Comet Encke struck the plains of Mare Imbrium,
making a crater about 3 meters wide.

Right:
The light curve of the May 2nd explosion in Mare Nubium. [Larger
image]

Now
that regular monitoring has begun, Cooke's group has already
found a second impact, the May 2nd event, in only 20 hours
of watching. This time, they believe, the impactor was a random
meteoroid, "a sporadic," from no particular comet
or asteroid.

"We've
made a good beginning," says Cooke, but much work remains.
He would like to observe all year long, watching the Moon
as it passes in and out of known meteoroid streams. "This
would establish a good statistical basis for planning [activities
on the Moon]."

Is
it safe to go moon walking during a meteor shower? How much
shielding does a lunar habitat need? Does the Moon have its
own meteor showers, unknown on Earth?